This invention relates to a method of manufacturing an information recording medium for use as a recording medium for an information processing apparatus, and a substrate therefor.
A magnetic disk is known as one of information recording media of the type. The magnetic disk comprises a substrate and a thin film such as a magnetic layer formed thereon. As the substrate, use has been made of an aluminum substrate or a glass substrate. In recent years, in response to the demand for high-density recording, the glass substrate is used at an increasing ratio because a gap between a magnetic head and the magnetic disk can be small as compared with the aluminum substrate.
Generally, the glass substrate increasingly used as described above is manufactured through chemical strengthening in order to increase the strength so that the glass substrate is resistant against a shock when loaded into a magnetic disk drive. In addition, in order to lower a flying height of the magnetic head as low as possible, the surface of the glass substrate is polished with high precision. Thus, high-density recording is realized.
On the other hand, the magnetic head is developed from a thin film head to a magnetoresistive head (MR head), further to a giant (large-sized) magnetoresistive head (GMR head) so as to meet the high-density recording.
As described above, the flying height must be lowered to achieve the high-density recording. To this end, it is essential that the surface of the magnetic disk has a high flatness. In addition, in case where the MR head is used, the surface of the magnetic recording medium must have a high flatness also from a problem of thermal asperity. The thermal asperity is a phenomenon that, if the surface of the magnetic disk has an abnormal protrusion, the MR head is affected by the protrusion to generate heat on the MR head and the resistance of the head fluctuates due to the heat to cause operation error in electromagnetic conversion. In this invention, the abnormal protrusion is a protrusion having a protrusion height specified by a surface roughness Rmax which satisfies Rmax/Raxe2x89xa720 when another surface roughness Ra is on the order between 0.1 and 0.7 nm, where Rmax is defined as a maximum height representative of a difference between a highest point and a lowest point and where Ra is representative of a center-line-mean roughness which will later be described.
Thus, in order to lower the flying height and to prevent the occurrence of the thermal asperity, there is an increasing demand for a high flatness of the surface of the magnetic disk. In order to achieve such a high flatness of the surface of the magnetic disk, a substrate surface must have a high flatness. However, the development in technology has reached a stage in which the high density recording of the magnetic disk is no longer achieved only by polishing the substrate surface with high precision. In other words, even if the substrate is polished with high precision, a high flatness is not obtained in case where foreign matter is adhered to the substrate. In fact, the removal of the foreign matter has been carried out. At a recent level of the high-density recording, however, fine foreign matter on the substrate is a problem although it has been allowed previously.
On the other hand, the above-mentioned glass substrate having a high flatness is obtained through a polishing step using abrasive grains of cerium oxide and another polishing step using colloidal silica grains. Recently, it is revealed that the glass substrate for a magnetic disk is useful not only as a 2.5-inch glass substrate mounted on a hard disk of a notebook-type personal computer but also for a desktop-type personal computer. Therefore, a demand for 3-inch and 3.5-inch glass substrates is rapidly increasing. Under the circumstances, it is disadvantageous in an economical aspect to perform the polishing steps using the different abrasive grains. It is therefore required to provide a method of manufacturing a glass substrate, which is capable of in achieving a high flatness comparable to the prior art by the use of cerium oxide grains. However, the above-mentioned foreign matter is not ignorable after the polishing step by the use of the cerium oxide grains.
The foreign matter of the type can not be removed by a typical cleaning step (ultrasonic cleaning or scrub cleaning by a neutral detergent, water, IPA (isopropyl alcohol), or the like) following a precision polishing step using abrasive grains such as cerium oxide and providing a surface roughness Ra of 1.0 nm or less. The foreign matter has a protruding height (Rmax) on the order of 10 nm. The foreign matter of the type is produced as a result of the precision polishing step using the abrasive grains such as cerium oxide, and is believed to mainly comprise polishing residue (protrusion). There is a recent demand for a glass substrate with a ultraflat or ultrasmooth surface having a surface roughness of 3 nm or less as Rmax and 0.3 nm or less as Ra. In such a situation, the polishing residue mentioned above is a serious problem. In case where a thin film such as a magnetic film is deposited on the glass substrate with the polishing residue adhered thereto, a projecting portion is formed on the surface of the magnetic disk to inhibit the lowering of the flying height and the prevention of occurrence of the thermal asperity.
It is an object of this invention to prevent polishing residue of the type from being left on a glass substrate.
It is another object of this invention to manufacture an information recording medium at a high yield by the use of a glass substrate from which polishing residue as a film defect is removed.
Methods according to this invention are as follows.
(1) A method of manufacturing a glass substrate for an information recording medium, comprising the steps of:
polishing a principal surface of the glass substrate; and
carrying out, subsequently to the step of polishing the principal surface, sulfuric acid-cleaning of at least the principal surface so that the glass substrate has surface roughnesses Ra and Rmax satisfying Ra=0.1-0.7 nm and Rmax/Ra less than 20 as measured by an inter-atomic force microscope (AFM), where Ra is representative of a center-line-mean roughness and where Rmax is defined as a maximum height representative of a difference between a highest point and a lowest point.
(2) A method of manufacturing an information recording medium, comprising the step of forming at least a recording layer on the glass substrate for the information recording medium that is manufactured by the method as mentioned in the paragraph (1).
(3) A method of manufacturing a glass substrate for an information recording medium, comprising:
a polishing step of polishing a principal surface of the glass substrate;
a chemical strengthening step of strengthening the glass substrate by replacing a part of ions contained in the glass substrate by substitute ions greater in ion-diameter than the part of ions; and
a sulfuric acid-cleaning step of cleaning at least the principal surface with sulfuric acid after the polishing step and before the chemical strengthening step.
(4) A method of manufacturing an information recording medium, comprising the step of forming at least a recording layer on the glass substrate for the information recording medium that is manufactured by the method as mentioned in the paragraph (3).
(5) A method of manufacturing a glass substrate for an information recording medium, comprising the steps of:
polishing a principal surface of the glass substrate;
cleaning at least the principal surface with an alkaline cleaning solution subsequently to the step of polishing the principal surface; and
cleaning at least the principal surface with sulfuric acid subsequently to the step of cleaning at least the principal surface with the alkaline cleaning solution,
(6) A method of manufacturing an information recording medium, comprising the step of forming at least a recording layer on the glass substrate for the information recording medium that is manufactured by the method as mentioned in the paragraph (5).
(7) A method of manufacturing a glass substrate for an information recording medium, comprising the step of polishing a principal surface of the glass substrate, wherein:
the step of polishing the principal surface uses abrasive grains made of cerium oxide;
the method further comprising the step of cleaning at least the principal surface with sulfuric acid subsequently to the step of polishing the principal
(8) A method of manufacturing an information recording medium, comprising the step of forming at least a recording layer on the glass substrate for the information recording medium that is manufactured by the method as mentioned in the paragraph (7).